Non-aqueous electrolyte secondary battery

ABSTRACT

A non-aqueous electrolyte secondary battery including: a positive electrode having a positive electrode material mixture containing a composite lithium oxide; a negative electrode; a polyolefin separator; a non-aqueous electrolyte; and a heat-resistant insulating layer interposed between the positive and negative electrodes. The positive electrode material mixture has an estimated heat generation rate at 200° C. of not greater than 50 W/kg. The positive electrode and the negative electrode are wound together with the separator and the heat-resistant insulating layer interposed therebetween.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 11/315,189 filed Dec. 23, 2005 which in turn claims the benefitof Japanese Application No. 2004-374200 filed Dec. 24, 2004, thedisclosures of which Applications are incorporated by reference hereinin their entirety.

FIELD OF THE INVENTION

The present invention relates to a non-aqueous electrolyte secondarybattery, and more particularly to an improvement of safety thereof.

BACKGROUND OF THE INVENTION

In recent years, there has been a rapid advancement in the developmentof potable and cordless electronic devices. With this development, thecommercialization of non-aqueous electrolyte secondary batteries havinga high voltage and a high energy density as the power sources fordriving these electronic devices is proceeding.

The positive electrode for non-aqueous electrolyte secondary batteriesusually contains a composite lithium oxide having a highoxidation-reduction potential. The most commonly used composite lithiumoxides are lithium cobalt oxide, lithium nickel oxide and lithiummanganese oxide. The negative electrode for non-aqueous electrolytesecondary batteries usually contains a carbon material. Non-aqueouselectrolyte secondary batteries also contain a non-aqueous electrolyteprepared by dissolving a lithium salt in a non-aqueous solvent. As thelithium salt, LiClO₄ and LiPF₆ are typically used. Between the positiveelectrode and the negative electrode is disposed a separator. Theseparator is usually a microporous film made of a polyolefin resinmaterial.

In the event where a short circuit of a relatively low resistance iscaused inside a battery by some kind of factor, a large current flowsintensively through the shorted point. The battery is thus heated andmay reach a excessively high temperature. In order to prevent suchphenomenon, various precautions are taken to provide safe batteries.

In the production aspect, the control of metal powders and the controlof dust in the production atmosphere are conducted to prevent theintrusion of foreign matter into a battery. Another way is to protectthe exposed portion(s) of current collectors having low resistance with,for example, an insulating tape so as to minimize the risk of aninternal short-circuit.

Separators having shut-down function are also used. In the event were ashort-circuit of a relatively low resistance occurs inside a battery,the pores of a separator having shut-down function close at about 135°C. so as to cut off an ion current. The short-circuit current is thuscut off, and heat generation stops. The surface temperature of thebattery, however, increases to about 120° C.

In order to prevent an internal short-circuit, for example, JapaneseLaid-Open Patent Publication No. Hei 7-220759 proposes to form, on anelectrode, a 0.1 to 200 μm thick layer composed of an inorganic particleand a resin binder. An internal short-circuit often results from partialseparation of a material from an electrode during the production of thebattery. The above-mentioned publication is intended to improve theproduction yield by preventing such internal short-circuit.

Japanese Laid-Open Patent Publication No. Hei 9-208736 proposes to applya heat-resistant resin (e.g., aramid) to a separator. This publicationis also intended to prevent an internal short-circuit.

According to the above proposals, it is possible to reduce heatgeneration to a certain extent in the event where an internalshort-circuit occurs locally. However, in nail penetration test which isa test to assess safety by simulating possible multiple simultaneousinternal short-circuits that can cause damage to a battery, multipleshorted points occur simultaneously. Under such severe short-circuitconditions, the batteries disclosed by the above publications cannotalways reduce heat generation, and the batteries may reach anexcessively high temperature.

When a typical lithium ion battery, which comprises a positive electrodecontaining lithium cobalt oxide, a negative electrode containinggraphite and a separator made of a polyethylene microporous film, issubjected to nail penetration test, the battery temperature increasesuntil the separator exerts its shutdown function, and the surfacetemperature of the battery reaches about 120° C. This temperatureincrease is due to Joule heat generated inside the battery byshort-circuit current.

Because the separator's shutdown function cuts the short-circuitcurrent, heat generation is reduced before the battery temperaturereaches more than 120° C. The safety evaluation standards for nailpenetration test and crush test established by Japan Storage BatteryAssociation require that batteries should be free from smoke, ignitionand rupture, and no standard is set as to battery temperature. As such,even if the surface temperature of a battery reaches about 120° C., aslong as the shutdown function works and heat generation is reduced, thebattery is deemed to be satisfying the safety standards.

However, even if the safety standards are satisfied, when the surfacetemperature of a battery increases to about 120° C., the temperature ofthe electronic device containing the battery also increases, which maycause deformation of the body of the electronic device and impair safetyof the electronic device. Under the circumstances, there is a desire forbatteries with further enhanced safety and reliability. Morespecifically, there is a very strong desire to reduce the maximumbattery surface temperature to 80° C. or lower even when an internalshort-circuit occurs.

In the case of the battery disclosed by Japanese Laid-Open PatentPublication No. Hei 7-220759 having a 0.1 to 200 μm thick layer composedof an inorganic particle and a resin binder formed on an electrode, thebattery surface temperature can reach as high as 80° C. or higher in thenail penetration test.

Similarly, in the case of the battery disclosed by Japanese Laid-OpenPatent Publication No. Hei 9-208736 having a separator with aheat-resistant resin applied thereon, the battery surface temperaturecan reach as high as 80° C. or higher in the nail penetration test.

Therefore, according to the above-mentioned publications, batterysurface temperature cannot always be reduced to not greater than 80° C.when multiple shorted points occur simultaneously. The reason that thebattery surface temperature increases to higher than 80° C. in the nailpenetration test can be explained as follows.

In the case where internal short-circuit occurs discretely, the layercomposed of an inorganic particle and a resin binder as well as theheat-resistant resin prevent the shorted point from expanding. Becausethe shorted point burns out instantly due to its self heat-generation,the short circuit condition lasts only for 0.1 to 0.5 seconds.Thereafter, the electrical insulation recovers. Once the short-circuitcurrent is cut off, the generated heat spreads out through the entirebattery, and therefore the battery temperature does not increase to ahigh temperature. The rest (i.e., the area other than the shorted point)has a relatively low temperature, so that the heat spreads out smoothly.

In the case of the nail penetration test, in contrast, multiple shortedpoints occur simultaneously in a battery. Under such severeshort-circuit conditions, heat is generated not only by the internalshort-circuits, but also by thermal decomposition reaction of thepositive electrode active material which generates heat continuously.Accordingly, the heat release rate at which heat spreads lags behind theheat generation rate, and thermal decomposition reaction of the positiveelectrode active material proceeds increasingly. This causes theseparation or burn-out of the positive electrode active material nearthe shorted points. The positive electrode current collector (e.g.,aluminum foil) is thus exposed to create another shorted point. As aresult, such internal short-circuit condition is maintained, and thesurface temperature of the battery increases until it reaches thetemperature at which the separator exerts its shut-down function,namely, about 120° C. In the case of discrete internal short-circuit,the thermal decomposition reaction of the positive electrode activematerial does not proceed. As such, the separation or burn-out of thepositive electrode active material does not occur, and thus additionalshorted point is not created.

BRIEF SUMMARY OF THE INVENTION

In view of the above, the present invention is intended to improve theabove situation and provides a non-aqueous electrolyte secondary batteryhaving a higher level of safety than conventional ones while maintaininga high energy density.

The present invention relates to a non-aqueous electrolyte secondarybattery comprising: a positive electrode comprising a positive electrodematerial mixture containing a composite lithium oxide and a positiveelectrode current collector carrying the positive electrode materialmixture; a negative electrode comprising a material capable of absorbingand desorbing lithium; a separator interposed between the positiveelectrode and the negative electrode, the separator comprising apolyolefin resin; a non-aqueous electrolyte; and a heat-resistantinsulating layer interposed between the positive electrode and thenegative electrode, wherein the positive electrode and the negativeelectrode are wound together with the separator and the heat-resistantinsulating layer interposed therebetween, wherein the positive electrodematerial mixture has an estimated heat generation rate at 200° C of notgreater than 50 W/kg.

In one embodiment of the present invention, the heat-resistantinsulating layer has a thickness of 1 μm or more and 15 μm or less.

In another embodiment of the present invention, the heat-resistantinsulating layer has a thickness of 1 μm or more and 5 μm or less.

The estimated heat generation rate is, for example, determined by thesteps of: (i) determining a relation between absolute temperature T andheat generation rate V of the positive electrode material mixture usingan accelerating rate calorimeter (ARC) (i.e. a runaway reactionmeasuring apparatus); (ii) plotting a relation between the inverse ofabsolute temperature T as an X coordinate and the logarithm of heatgeneration rate V as a Y coordinate according to the Arrhenius law;(iii) obtaining an approximate straight line fitted to the plottedpoints in a heat generation temperature range of T<200° C (473 K); and(iv) extrapolating the obtained approximate straight line to thetemperature axis at T=200° C. (473 K).

As used herein, the heat generation temperature range means a range inwhich the absolute value of the negative gradient of the approximatestraight line is the largest in a plot according to the Arrhenius law.Accordingly, in the step (iii), the approximate straight line should bedrawn such that the absolute value of the negative gradient is thelargest. The extrapolation is a method for estimating a value that fallsoutside a range of known values based on the known values in data, andit is widely used in various fields.

The composite lithium oxide is preferably one of the following.

(i) A composite lithium oxide having a composition represented by ageneral formula (1) : Li_(a)M_(b)Me_(c)O₂, where element M is at leastone selected from the group consisting of Al, Mn, Sn, In, Fe, Cu, Mg,Ti, Zn, Zr and Mo, and element Me is at least one selected from thegroup consisting of Ni and Co, and where the general formula (1)satisfies: 0.9<a<1.2; 0.02≦b≦0.5; 0.5≦c≦0.98; and 0.95≦b+c≦1.05.

(ii) A composite lithium oxide having a composition represented by ageneral formula (2) : Li_(a)M_(b)Ni_(c)Co_(e)O2, where element M is atleast one selected from the group consisting of Al, Mn, Sn, In, Fe, Cu,Mg, Ti, Zn, Zr and Mo, and where the general formula (2) satisfies:0.9<a<1.2; 0.02≦b≦0.5; 0.1≦d≦0.5; 0.1≦e≦0.5; and 0.95≦b+d+e≦1.05. Morepreferably, the general formula (2) satisfies: 0.15≦b≦0.4; 0.3≦d≦0.5;and 0.15≦e≦0.4.

(iii) A composite lithium oxide having any composition comprisingelement M, the element M being at least one selected from the groupconsisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr and Mo, and theelement M being distributed more in a surface portion of the compositelithium oxide than the inside of the composite lithium oxide. Similarly,in the composite lithium oxides represented by the general formula (1)and (2), the element M is preferably distributed more in a surfaceportion than the inside of the composite lithium oxides.

The composite lithium oxide has preferably been treated with an Sicompound represented by a general formula (3): X—Si—Y₃. In the formula(3), X includes a functional group reactive with the composite lithiumoxide, and Y includes a functional group comprising C, H, O, F or Si.

According to the present invention, the heat generation due to internalshort circuit and the chain of heat generation reaction are effectivelyprevented even under severe conditions where multiple shorted pointsoccur simultaneously. Because the maintenance of the short circuit isavoided, the maximum battery surface temperature can always bemaintained at not greater than 80° C. According to the presentinvention, it is possible to provide a non-aqueous electrolyte secondarybattery having a higher level of safety than conventional ones whilemaintaining a high energy density.

While the novel features of the invention are set forth particularly inthe appended claims, the invention, both as to organization and content,will be better understood and appreciated, along with other objects andfeatures thereof, from the following detailed description taken inconjunction with the drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 shows relations between the inverse of absolute temperature T andthe logarithm of heat generation rate V determined by an acceleratingrate calorimeter (ARC) for different positive electrode materialsaccording to the Arrhenius law.

FIG. 2 is a graph used to explain the measuring principle of ARC.

DETAILED DESCRIPTION OF THE INVENTION

A non-aqueous electrolyte secondary battery of the present inventioncomprises: a positive electrode comprising a positive electrode materialmixture containing a composite lithium oxide and a positive electrodecurrent collector carrying the positive electrode material mixture; anegative electrode comprising a material capable of absorbing anddesorbing lithium; a separator comprising a polyolefin resin; anon-aqueous electrolyte; and a heat-resistant insulating layerinterposed between the positive electrode and the negative electrode.

The heat-resistant insulating layer is, for example, formed on eitherthe positive electrode or the negative electrode on a surface thereoffacing the other electrode, but the arrangement of the heat-resistantinsulating layer is not limited thereto. The heat-resistant insulatinglayer may be formed on at least one surface of the positive electrode,or on at least one surface of the negative electrode, or on at least onesurface of the separator. Alternatively, it may be formed on at leastone surface of the positive electrode and at least one surface of thenegative electrode, or on at least one surface of the negative electrodeand at least one surface of the separator, or on at least one surface ofthe separator and at least one surface of the positive electrode.Alternatively, it may be formed on at least one surface of the positiveelectrode and at least one surface of the negative electrode and atleast one surface of the separator. The heat-resistant insulating layermay be in the form of a sheet independent of the positive electrode, thenegative electrode and the separator. However, the heat-resistantinsulating layer is desirably bonded or adhered to at least one surfaceof the positive electrode, to at least one surface of the negativeelectrode, or to at least one surface of the separator.

A feature of the present invention is that the estimated heat generationrate at 200° C. of the positive electrode material mixture is controlledto not greater than 50 W/kg. The estimated heat generation is, forexample, determined by the steps of: (i) determining a relation betweenabsolute temperature T and heat generation rate V of the positiveelectrode material mixture using an accelerating rate calorimeter; (ii)plotting a relation between the inverse of absolute temperature T (Xcoordinate) and the logarithm of heat generation rate V (Y coordinate)according to the Arrhenius law; (iii) obtaining an approximate straightline fitted to the plotted points in a heat generation temperature rangeof T<200° C. (473 K); and (iv) extrapolating the obtained approximatestraight line to the temperature axis at T=200° C. (473 K).

When the positive electrode material mixture has an estimated heatgeneration rate at 200° C. determined by the extrapolation of 50 W/kg orless, the contribution of the heat-resistant insulating layer to safetyis significantly enhanced particularly under severe conditions wheremultiple shorted points occur simultaneously. The present invention hasbeen accomplished based on the above finding, and provides an extremelyhigher level of safety than conventional batteries.

The estimated heat generation rate at 200° C. of the positive electrodematerial mixture can be controlled to not greater than 50 W/kg by usingpositive electrode materials shown below.

As an effective positive electrode material to control the estimatedheat generation rate to not greater than 50 W/kg, a composite lithiumoxide having a composition represented by a general formula (1):Li_(a)M_(b)Me_(c)O₂, where element M is at least one selected from thegroup consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr and Mo, andelement Me is at least one selected from the group consisting of Ni andCo, and where the general formula (1) satisfies: 0.9<a<1.2, 0.02≦b≦0.5;0.5≦c≦0.98; and 0.95≦b+c≦1.05 can be used.

From the viewpoint of reducing the estimated heat generation rate, theelement M is preferably Mn, Al or Mg, most preferably Mn, although allthe above-listed elements (i.e., Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zrand Mo) have the effect of reducing the estimated heat generation rate.

In the general formula (1), the value of “a” is the initial value whichfluctuates during charge/discharge of the battery. The initial valueshould be actually the same as the value of “a” of the composite lithiumoxide contained in a battery in a discharged state. Typically, thecomposite lithium oxide immediately after the production has a value of“a” of 1.

When the value of “b” is less than 0.02, the effect of the element Mcannot be observed. When the value of “b” exceeds 0.5, the capacitydecreases significantly.

When the value of “c” is less than 0.5, it is difficult to ensure acertain amount of capacity. When the value of “c” exceeds 0.98, theeffect of reducing the estimated heat generation rate cannot beobtained.

The general formula (1) satisfies 0.95≦b+c≦1.05. In the initialcondition immediately after the production (i.e., in a fresh conditionbefore initial charge/discharge is performed), the value of “b+c” istypically 1, but it is not necessarily exactly 1. As long as the valueof “b+c” falls within the range: 0.95≦b+c≦1.05, the value of “b+c” canbe deemed as 1 (b+c=1).

Another effective positive electrode material to control the estimatedheat generation rate to not greater than 50 W/kg is a composite lithiumoxide having a composition represented by a general formula (2) :Li_(a)M_(b)Ni_(c)Co_(e)O2, where element M is at least one selected fromthe group consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr and Mo,and where the general formula (2) satisfies: 0.9<a<1.2; 0.02≦b≦0.5;0.1≦d≦0.5; 0.1≦e≦0.5; and 0.95≦b+d+e≦1.05. More preferably, the generalformula (2) satisfies: 0.15≦b≦0.4; 0.3≦d≦0.5; and 0.15≦e≦0.4.

In the general formula (2) also, the value of “a” is the initial valueso that it fluctuates during charge/discharge. When the value of “b” isless than 0.02, the effect of the element M cannot be observed. When thevalue of “b” exceeds 0.5, the capacity decreases significantly.

When the value of “d” is less than 0.1, the effect obtained by theaddition of Ni (e.g., the effect of improving theoretical capacity)drops. When the value of “d” exceeds 0.5, the battery voltage lowers andcycle life characteristic also deteriorates.

When the value of “e” is less than 0.1, the effect obtained by theaddition of Co (e.g., the effect of increasing voltage) drops. When thevalue of “e” exceeds 0.5, the utilization efficiency of the positiveelectrode lowers.

The general formula (2) satisfies 0.95≦b+d+e≦1.05. In the initialcondition immediately after the production (i.e., in a fresh conditionbefore initial charge/discharge is performed), the value of “b+d+e” istypically 1, but it is not necessarily exactly 1. As long as the valueof “b+d+e” falls within the range: 0.95≦b+d+e≦1.05, the value of “b+d+e”can be deemed as 1 (b+d+e=1).

A specific example of the positive electrode material represented by theformula (2) and effective to control the estimated heat generation rateat 200° C. to not greater than 50 W/kg is LiMn_(b)Ni_(d)CO_(e)O₂, where0.15≦b≦0.35, 0.3≦d≦0.5, and 0.25≦e≦0.35.

Another effective positive electrode material to control the estimatedheat generation rate to not greater than 50 W/kg is a composite lithiumoxide having any composition comprising element M, the element M beingat least one selected from the group consisting of Al, Mn, Sn, In, Fe,Cu, Mg, Ti, Zn, Zr and Mo, and the element M being distributed more in asurface portion of the composite lithium oxide than the inside of thecomposite lithium oxide.

Such positive electrode material can be obtained by applying a compoundcontaining the element M (e.g., a nitrate or sulfate) onto the surfaceof a composite lithium oxide having any composition (e.g., a compositelithium oxide represented by the general formula (1) or (2)) anddiffusing the element M into the composite lithium oxide. As an example,a mixture prepared by mixing a composite lithium oxide with a smallamount of compound containing the element M is baked at an appropriatetemperature, whereby the element M diffuses from the surface into theinside of the composite lithium oxide. As a result, a composite lithiumoxide in which the element M is distributed more in the surface portionthan the inside can be obtained. Alternatively, a liquid dissolving ordispersing a compound containing the element M is mixed with a lithiumcomposite oxide, after which the liquid component is removed to obtain acomposite lithium oxide carrying the element M. By baking thethus-obtained composite lithium oxide at an appropriate temperature(e.g., 300 to 700° C.), the element M can be diffused from the surfaceinto the inside of the composite lithium oxide.

The compounds containing the element M have a significant effect inreducing the estimated heat generation rate at 200° C. However, as theamount of compound containing the element M to be added to a compositelithium oxide is increased, the utilization efficiency of the positiveelectrode decreases, and the energy density of the resulting batterydecreases. Because the heat generation reaction of the positiveelectrode takes place on the surface of the active material particles,by having more element M on the surface of the active materialparticles, heat generation can be reduced efficiently withoutsignificant decrease of the utilization efficiency of the positiveelectrode. In other words, the addition of only a small amount ofelement M can reduce the estimated heat generation rate because theelement M is distributed intensely on the surface of the active materialparticles.

The amount of the compound containing the element M is preferablyadjusted such that the amount of the element M is 0.0001 mol to 0.05 molper 1 mol of the composite lithium oxide.

Another effective positive electrode material to control the estimatedheat generation rate to not greater than 50 W/kg is a composite lithiumoxide treated with an Si compound represented by a general formula (3):X—Si—Y₃. In the formula (3), X includes a functional group reactive withthe composite lithium oxide, and Y includes a functional groupcomprising C, H, O, F or Si. By reforming the surface of the compositelithium oxide with an Si compound, the heat generation reaction thattakes place on the surface of the active material particles can bereduced, and therefore the estimated heat generation rate can bereduced. The composite lithium oxide treated with an Si compound doesnot significantly affect the utilization efficiency of the positiveelectrode.

The composite lithium oxide is preferably treated with a silane couplingagent represented by X—Si—Y₃, for example. The method for treating thecomposite lithium oxide with a silane coupling agent represented byX—Si—Y₃ is not specifically limited. As an example, a silane couplingagent is mixed with water to prepare a mixture. The obtained mixture isthen mixed with a composite lithium oxide, followed by drying. Theconcentration of the silane coupling agent in the mixture of the silanecoupling agent and water is preferably about 0.01 wt % to 5 wt %, morepreferably, 0.1 wt % to 3 wt %. The amount of the silane coupling agentis preferably 0.001 to 0.5 parts by weight per 100 parts by weight ofthe composite lithium oxide, more preferably 0.01 to 0.1 parts byweight.

Examples of the silane coupling agent include vinyl triethoxy silane,vinyl trimethoxy silane, vinyl trichloro silane, vinyltris(2-methoxyethoxy)silane, γ-methacryloxypropyl trimethoxy silane,γ-methacryloxypropyl triethoxy silane, γ-aminopropyl triethoxy silane,γ-aminopropyl trimethoxy silane, N-β-(aminoethyl)-γ-aminopropyltrimethoxy silane, N-γ-(aminoethyl)-γ-aminopropyl triethoxy silane,γ-ureidopropyl triethoxy silane, γ-ureidopropyl trimethoxy silane,β-(3,4-epoxycyclohexyl)ethyl trimethoxy silane,β-(3,4-epoxycyclohexyl)ethyl triethoxy silane, γ-glycidoxypropyltrimethoxy silane, γ-glycidoxypropyl triethoxy silane, γ-mercaptopropyltrimethoxy silane, γ-mercaptopropyl triethoxy silane, γ-chloropropyltrimethoxy silane, γ-chloropropyl triethoxy silane, methyl triethoxysilane, methyl trimethoxy silane, phenyl triethoxy silane, and phenyltrimethoxy silane. Among them, particularly preferred are vinyltriethoxy silane, vinyl trimethoxy silane, vinyltris(2-methoxyethoxy)silane, γ-methacryloxypropyl trimethoxy silane,γ-methacryloxypropyl triethoxy silane, γ-aminopropyl triethoxy silane,γ-aminopropyl trimethoxy silane, N-β-(aminoethyl)-γ-aminopropyltrimethoxy silane, N-β-(aminoethyl)-γ-aminopropyl triethoxy silane,γ-ureidopropyl triethoxy silane, γ-ureidopropyl trimethoxy silane,β-(3,4-epoxycyclohexyl)ethyl trimethoxy silane,β-(3,4-epoxycyclohexyl)ethyl triethoxy silane, γ-glycidoxypropyltrimethoxy silane, and γ-glycidoxypropyl triethoxy silane.

The heat-resistant insulating layer comprises, for example, an inorganicoxide filler and a resin component. The inorganic oxide filler has highthermal resistance. Accordingly, the heat-resistant insulating layer canretain its high mechanical strength even when the battery temperaturereaches relatively a high level. The resin component contained in theheat-resistant insulating layer may be any resin component, but a resincomponent having high thermal resistance is preferably used because aheat-resistant insulating layer having excellent thermal resistance canbe obtained.

For example, the heat-resistant insulating layer comprises an inorganicoxide filler and a binder (resin component) or comprises aheat-resistant resin (resin component), but there is no particularlimitation. When the heat-resistant insulating layer comprises aninorganic oxide filler and a binder, it has a relatively high mechanicalstrength and hence a high durability. The main component of theheat-resistant insulating layer comprising an inorganic oxide filler anda binder is the inorganic oxide filler. For example, the inorganic oxidefiller constitutes not less than 80 wt %, preferably not less than 90 wt%, of the heat-resistant insulating layer. When the heat-resistantinsulating layer comprises a heat-resistant resin, the heat-resistantresin constitutes, for example, more than 20 wt % of the heat-resistantinsulating layer.

The heat-resistant insulating layer comprising a heat-resistant resinhas a higher flexibility than the heat-resistant insulating layercomposed mainly of an inorganic oxide filler, since heat-resistantresins are more flexible than inorganic oxide fillers. Thus, theheat-resistant insulating layer comprising a heat-resistant resin ismore likely to conform to expansion and contraction of the electrodeplates during charge/discharge, so that it is capable of retaining itshigh heat resistance and provides a high safety upon nail penetration.

The heat-resistant insulating layer comprising a heat-resistant resincan contain, for example, less than 80 wt % of an inorganic oxidefiller. When the heat-resistant insulating layer contains an inorganicoxide filler, it has a good balance between flexibility and durability.Heat-resistant resins contribute to the flexibility of theheat-resistant insulating layer, while inorganic oxide fillers havinghigh mechanical strength contribute to the durability. The inclusion ofan inorganic oxide filler in the heat-resistant insulating layerimproves the high output characteristics of the battery. This isprobably because flexibility and durability produce a synergistic effectof optimizing the pore structure of the heat-resistant insulating layer,although the detailed reason is not clear. In terms of ensuring goodhigh output characteristics, it is preferred that the heat-resistantinsulating layer comprising a heat-resistant resin contain 25 wt % to 75wt % of an inorganic oxide filler.

The resin component (binder or heat-resistant resin) of theheat-resistant insulating layer preferably has a thermal decompositiontemperature of not less than 250° C. and does not deform significantlyat a high temperature. That is, the resin component of theheat-resistant insulating layer is preferably amorphous ornon-crystalline. Also, the resin component preferably has a thermaldeformation temperature or glass transition temperature (Tg) of not lessthan 250° C.

The thermal decomposition temperature, thermal deformation temperatureor glass transition temperature of the resin component can be measuredby differential scanning calorimetry (DSC) orthermogravimetry-differential thermal analysis (TG-DTA). The temperatureat which the weight starts to change in the TG-DTA corresponds tothermal decomposition temperature. The inflection point of theendothermic shift in the DSC corresponds to thermal deformationtemperature or glass transition temperature.

Preferred examples of the binder contained in the heat-resistantinsulating layer include fluorocarbon resins such as polyvinylidenefluoride (PVDF), and rubbery polymers containing an acrylonitrile unit(modified acrylonitrile rubber). They may be used singly or in anycombination of two or more. Among them, particularly preferred arerubbery polymers containing an acrylonitrile unit because they haveappropriate thermal resistance, elasticity and binding capability.

Preferred examples of the heat-resistant resin contained in theheat-resistant insulating layer include polyamide resins such asaromatic polyamide (aramid), polyimide resins, and polyamide imideresins. They may be used singly or in any combination of two or more.

Preferably, the heat-resistant insulating layer comprising an inorganicoxide filler and a binder is formed on or bonded to at least one surfaceof the negative electrode, and more preferably, this layer is formed onor bonded to both surfaces of the negative electrode. Preferably, theheat-resistant insulating layer comprising a heat-resistant resin isformed on or bonded to at least one surface of the separator. Since theheat-resistant insulating layer is relatively brittle, it is morepreferred that this layer be formed on or bonded to only one surface ofthe separator. When the heat-resistant insulating layer comprising aheat-resistant resin is formed on only one surface of the separator, theratio of the thickness A of the separator to the thickness B of theheat-resistant insulating layer (A/B ratio) satisfies, for example, therelation: 3≦A/B≦12, or 4≦A/B≦6, in terms of preventing the breakage ofthe heat-resistant insulating layer.

Examples of the inorganic oxide filler include alumina (Al₂O₃), titania(TiO₂), silica (SiO₂), zirconia and magnesia. They may be used singly orin any combination of two or more. Among them, particularly preferredare alumina (α-alumina in particular) and magnesia because they arestable, easy to handle and less costly.

The average particle size (median size: D50) of the inorganic oxidefiller is not specifically limited. Preferably, the inorganic oxidefiller has an average particle size of 0.1 μm to 5 μm, more preferably,0.2 μm to 1.5 μm.

When the heat-resistant insulating layer comprises an inorganic oxidefiller and a binder, the content of the inorganic oxide filler ispreferably not less than 50 wt % and not greater than 99 wt %, morepreferably, not less than 90 wt % and not greater than 99 wt %. When thecontent of the inorganic oxide filler is less than 50 wt %, the amountof the resin component is excessively large, the control of the porestructure among the filler particles might be difficult. Conversely,when the content of the inorganic oxide filler exceeds 99 wt %, theamount of the resin component is excessively small, which might impairthe mechanical strength of the heat-resistant insulating layer or theadhesion of the heat-resistant insulating layer to electrode surface orseparator surface.

The thickness of the heat-resistant insulating layer is not specificallylimited. In order for the heat-resistant insulating layer to fully exertshort-circuit prevention function, or to fully insulate the shortedpoints while retaining the design capacity, the heat-resistantinsulating layer has, for example, a thickness of 1 μm or more and 15 μmor less. The thickness of the heat-resistant insulating layer comprisingan inorganic oxide filler and a binder is, for example, 3 to 15 μm, or 3to 8 μm. The thickness of the heat-resistant insulating layer comprisinga heat-resistant resin is, for example, 1.5 to 7 μm, or 1.7 to 6.7 μm.If the heat-resistant insulating layer is too thick, it may break whenthe electrodes are wound, since the heat-resistant insulating layer isbrittle. However, if the heat-resistant insulating layer is too thin, ithas poor strength, so it may break.

In the present invention, any type of conventional separators can beused. For example, a monolayer separator made of polyolefin resin suchas polyethylene or polypropylene, or a multilayer separator made ofpolyolefin resin can be used. A preferred thickness of the separator is,but not limited to, about 15 μm to 25 μm.

The positive electrode material mixture comprises an active materialcomprising a composite lithium oxide as the essential component.Optionally, the positive electrode material mixture further comprises abinder, a conductive material, etc. Examples of the binder for thepositive electrode include polytetrafluoroethylene (PTFE), modifiedacrylonitrile rubber particles, and PVDF. They may be used singly or inany combination of two or more. PTFE and modified acrylonitrile rubberparticles are preferably combined with carboxymethyl cellulose,polyethylene oxide or modified acrylonitrile rubber for use. They serveas a thickener in the paste composed of the positive electrode materialmixture and a liquid component. As the conductive material for thepositive electrode, acetylene black, ketjen black or any graphite can beused. They may be used singly or in any combination of two or more. Theamount of the binder contained in the positive electrode materialmixture is preferably 0.1 to 5 parts by weight per 100 parts by weightof the active material. The amount of the conductive material containedin the positive electrode material mixture is preferably 1 to 10 partsby weight per 100 parts by weight of the active material.

For producing a negative electrode comprising a carbon material or alloymaterial, any conventional material for negative electrodes can be used.Examples of the carbon material include any natural graphite and anyartificial graphite. Examples of the alloy material include a siliconalloy and a tin alloy. The carbon material and the alloy material may becombined. The negative electrode may further contain a binder, aconductive material, etc. As the binder and the conductive material forthe negative electrode, those listed above for the positive electrodecan be used.

The non-aqueous electrolyte is preferably prepared by dissolving alithium salt as the solute in a non-aqueous solvent. The lithium saltand the non-aqueous solvent are not specifically limited. Preferredexamples of the lithium salt include LiPF₆ and LiBF₄. Preferred examplesof the non-aqueous solvent include ethylene carbonate, propylenecarbonate, dimethyl carbonate, diethyl carbonate, and methyl ethylcarbonate. The non-aqueous solvent is preferably used in combination oftwo or more, rather than alone. Preferably, the non-aqueous solventcontains, as an additive, vinylene carbonate, vinylethylene carbonate orcyclohexylbenzene.

The present invention will be described below in further detail withreference to experiments and examples, but it should be understood thatthe scope of the present invention is not limited thereto.

Experiment 1

Measurement of Temperature of Shorted Point

Ten different 18650 type cylindrical non-aqueous electrolyte secondarybatteries each having a diameter of 18 mm and a height of 65 mm wereproduced. For the production, lithium cobalt oxide (LiCoO₂) was used asthe positive electrode active material. A heat-resistant insulatinglayer comprising an inorganic oxide filler and a resin component wasformed on the negative electrode surface. The produced batteries weresubjected to nail penetration test to check the temperature increase ofa shorted point 0.5 second after the penetration of a nail.

In this experiment, a thermocouple was attached to the surface of eachbattery. A nail was penetrated near the thermocouple, and the surfacetemperature of the battery was measured. The results are shown inTable 1. TABLE 1 Temperature of shorted point Battery (° C.) Cylindricalbattery 1 205 Cylindrical battery 2 203 Cylindrical battery 3 201Cylindrical battery 4 205 Cylindrical battery 5 216 Cylindrical battery6 203 Cylindrical battery 7 208 Cylindrical battery 8 204 Cylindricalbattery 9 210 Cylindrical battery 10 201

The 18650 type cylindrical non-aqueous electrolyte secondary batteriesused in EXPERIMENT 1 were each produced in the following procedure.

(i) Production of Positive Electrode

A positive electrode material mixture paste was prepared by mixing, in adouble blade mixer, 3 kg of lithium cobalt oxide (LiCoO₂), 1 kg of PVDF#1320 available from Kureha Chemical Industry Co., Ltd (anN-methyl-2-pyrrolidone (NMP) solution containing 12 wt % PVDF as thebinder), 90 g of acetylene black and an appropriate amount of NMP. Theprepared paste was then applied onto both surfaces of an aluminum foilhaving a thickness of 15 μm, followed by drying and rolling to formpositive electrode material mixture layers. An electrode plate whichcomprises the aluminum foil and the positive electrode material mixturelayers had a thickness of 160 μm. The obtained electrode plate was thencut into a size suitable for a battery case for 18650 type cylindricalbattery with a diameter of 18 mm and a height of 65 mm. Thereby, apositive electrode was produced.

(ii) Production of Negative Electrode

A negative electrode material mixture paste was prepared by mixing, in adouble blade mixer, 3 kg of artificial graphite, 75 g of BM-400Bavailable from Zeon Corporation, Japan (an aqueous dispersion containing40 wt % styrene-butadiene copolymer), 30 g of carboxymethylcellulose(CMC) as the thickener and an appropriate amount of water. The preparedpaste was then applied onto both surfaces of a copper foil having athickness of 10 μm, followed by drying and rolling to form negativeelectrode material mixture layers. An electrode plate comprising thecopper foil and the negative electrode material mixture layers had athickness of 180 μm. The obtained electrode plate was then cut into asize suitable for the same battery case as mentioned above. Thereby, anegative electrode was produced.

(iii) Preparation of Non-Aqueous Electrolyte

Lithium Hexafluorophosphate (LiPF₆) was dissolved in a solvent mixtureof ethylene carbonate and methyl ethyl carbonate at a volume ratio of1:3 at a LiPF₆ concentration of 1 mol/L to prepare a non-aqueouselectrolyte.

(iv) Preparation of Paste for Heat-Resistant Insulating Layer

A paste for the heat-resistant insulating layer was prepared by mixing,in a double blade mixer, 950 g of alumina (inorganic oxide filler)having an average particle size (median size) of 0.3 μm, 625 g ofBM-720H available from Zeon Corporation, Japan (an NMP solutioncontaining 8 wt % rubbery polymer containing an acrylonitrile unit asthe resin component) and an appropriate amount of NMP.

(v) Assembly of Battery

The paste for the heat-resistant insulating layer was applied onto bothsurfaces of the negative electrode, followed by drying to formheat-resistant insulating layers each having a thickness of 0.5 μm.

The positive electrode and the negative electrode having 0.5 μm thickheat-resistant insulating layers formed on both surfaces thereof werespirally wound with a separator interposed between the positive andnegative electrodes so as to form an electrode assembly. The separatorwas a 20 μm thick monolayer separator made of polyethylene resin. Theelectrode assembly was inserted into a battery case, and then 5.5 g ofthe non-aqueous electrolyte was injected into the battery case. Finally,the opening of the case was sealed. Thereby, a cylindrical non-aqueouselectrolyte secondary battery having a nominal capacity of 2000 mAh wasproduced.

Nail penetration test was performed as follows.

Each of the cylindrical batteries 1 to 10 was charged under thefollowing conditions:

constant current charge: 1400 mA (with an end-of-charge voltage of 4.25V); and

constant voltage charge: 4.25 V (with an end-of-charge current of 100mA).

The charged battery was pierced from the side thereof with a round ironnail having a diameter of 2.7 mm at a rate of 5 mm/sec. in anenvironment of 20° C. Then, the temperature of the shorted point (i.e.,the temperature of the area through which the nail penetrated) wasmeasured 0.5 second after the penetration of the nail.

As can be seen from Table 1, the temperature of the shorted pointincreased to 200° C. at the minimum for 0.5 second. It is generallyaccepted that lithium cobalt oxide in a charged state starts tothermally decompose at about 200° C. It can thus be surmised that underthe conditions where multiple shorted points occur simultaneously as inthe nail penetration test, the Joule heat continuously generates at theshorted points by current flow and therefore the decomposition reactionheat of the positive electrode active material occurs. This suggeststhat conventional batteries having a heat-resistant insulating layercomprising an inorganic oxide filler and a resin component cannot ensuresafety under the conditions where multiple simultaneous internalshort-circuits occur.

The foregoing illustrates that in order to ensure safety even under theconditions where multiple simultaneous internal short-circuits occur, itis very important to control the thermal stability of the positiveelectrode material. To be more specific, it is important not only toprevent short-circuiting by forming the heat-resistant insulating layer,but also to reduce thermal decomposition reaction of the positiveelectrode active material. Therefore, it can be concluded that thepositive electrode active material should be a material that does noteasily decompose even when the shorted point reaches a high temperatureof 200° C. or higher.

Experiment 2

Investigation of Positive Electrode Active Material

Because it has been shown that the heat-resistant insulating layer andthe thermal stability of the positive electrode active material are twovery important factors, an investigation was conducted on the thermalstability of the positive electrode material mixture. In thisexperiment, the estimated heat generation rate at 200° C. of positiveelectrode material mixtures containing positive electrode materials 1 to3 listed in Table 2 were measured. TABLE 2 Positive electrode material 1LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂ Positive electrode material 2LiAl_(0.05)Ni_(0.8)Co_(0.15)O₂ Positive electrode material 3 LiCoO₂

Using the materials 1 to 3 listed in Table 2 as the positive electrodeactive material, three different 18650 type cylindrical non-aqueouselectrolyte secondary batteries were produced in the same manner as inEXPERIMENT 1. The produced batteries (hereinafter referred to asbatteries 1A to 3A) were charged under the following conditions:

constant current charge: 1400 mA (with an end-of-charge voltage of 4.25V); and

constant voltage charge: 4.25 V (with an end-of-charge current of 100mA). The battery voltage 4.25 V corresponds to positive electrodepotential of 4.35 V vs Li metal.

The charged batteries 1A to 3A were disassembled in an atmosphere with adew point of not greater than −40° C., and then the positive electrodewas removed from each battery. The removed positive electrodes were cutinto sample pieces each with a size of 3 cm×6 cm. Each of the samplepieces was inserted into a cylindrical iron case (with a diameter of 8mm and a height of 65 mm) with the inner surface plated with Ni. Then,the opening of each case was sealed.

Subsequently, the positive electrode sample pieces enclosed in thehermetically sealed cylindrical cases were analyzed by an acceleratingrate calorimeter (ARC) under the conditions shown in Table 3 to obtaindata on the relation between absolute temperature T and heat generationrate V of the positive electrode material mixture. TABLE 3 Starttemperature 60° C. End temperature 460° C. Heat step (temperatureincrement) 20° C. Detection sensitivity 0.04° C./min. Wait time 15 min.Detection temperature difference 0.2° C.

In the ARC test, because a sample is held in an adiabatic atmosphere,the temperature increasing rate of the sample directly reflects the heatgeneration. The sample is given a temperature rise by stepwise heatinguntil the heat generation reaction has a heat generation rate over theset predetermined detection sensitivity. Once the ARC detects a heatgeneration rate over the set detection sensitivity, it measures the heatgeneration rate of the sample in an adiabatic atmosphere.

The terms listed in Table 3 are defined below with reference to FIG. 2.

Heat step (indicated by {circle around (1)} in FIG. 2): an increment ofthe atmospheric temperature increased stepwise within a range whereself-heating of the sample is not detected.

Detection sensitivity (indicated by {circle around (2)} in FIG. 2): asensitivity level at which self-heating of the sample is detected. Thesensitivity can be arbitrarily set according to the material used. Whena temperature increment by self-heating of the sample within detectiontime (Δt) is taken as ΔT, the sensitivity is expressed by ΔT/Δt.

Wait time (indicated by {circle around (3)} in FIG. 2): a time periodafter the atmospheric temperature is increased compulsory by a certaintemperature increment for allowing the sample temperature and theatmospheric temperature in the sample container to be stable. The waittime can be arbitrarily set.

Detection temperature difference (indicated by {circle around (4)} inFIG. 2): a temperature difference by which self-heating of the sample isdetected. Under the conditions where the detection temperaturedifference is 0.2° C. and the detection sensitivity is 0.04° C./min., ifa temperature increase at a rate of not less than 0.04° C./min.continues for 5 minutes (0.2/0.04), the ARC will recognize that heatgeneration has occurred.

Conventionally, thermoanalysis such as differential scanning calorimetry(DSC) and thermogravimetry-differential thermal analysis (TG-DTA) havebeen employed for thermal stability measurement of positive electrodeactive materials. However, the thermal stability measurements by DSC andTG-DTA suffer from a few problems, one of which is that heat generationrate or heat generation peak varies according to the measurementconditions (e.g., the temperature increase rate, the amount of thesample). Thus, the use of DSC and TG-DTA is not suitable for accuratemeasurement of the heat generation rate. Another problem is that, in theevent of internal short-circuit or the like, the temperature of ashorted point will instantly increase to 200° C. or higher. Therefore,heat that would have been generated at temperatures of less than 200° C.will also be generated simultaneously. The DSC and TG-DTA measurements,however, cannot predict the heat generation rate of a heat generationreaction at different temperatures. On the other hand, in the ARC test,because the sample is held in an adiabatic atmosphere, the temperatureincreasing rate of the sample indicates the heat generation rate as itis. For this reason, the ARC test is particularly effective in measuringthe reaction rate of an exothermic reaction. In view of the above, inthe present invention, an accelerating rate calorimeter (ARC) was usedfor measuring the thermal stability of the positive electrode materialmixtures at the time of internal short-circuit.

The data obtained by ARC was plotted according to the Arrhenius law asshown in FIG. 1. More specifically, the relation between the inverse ofabsolute temperature T (X coordinate) and the logarithm of heatgeneration rate V (Y coordinate) was plotted. An approximate straightline was drawn such that it fits to a set of the plotted points. Eachpoint showed the heat generation rate of a chemical reaction. Byextrapolating the approximate straight line to a certain temperatureaxis, it is possible to estimate the heat generation rate outside therange of the temperature at which heat generation is actually observed.In this experiment, as shown in FIG. 1, a straight line was drawn to befitted to the plotted points in the heat generation range of T<200° C.(473 K). Then, the straight line was extrapolated to the temperatureaxis at T=200° C. (473 K) to determine an estimated heat generationrate. The estimated heat generation rates thus obtained are shown inTable 4. TABLE 4 Positive electrode material 1LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂  10 W/kg Positive electrode material 2LiAl_(0.05)Ni_(0.8)Co_(0.15)O₂  100 W/kg Positive electrode material 3LiCoO₂ 2500 W/kg

The batteries 1A to 3A were subjected to the same nail penetration testas in EXPERIMENT 1. As a result, only in the battery 1A containing thepositive electrode material 1 (LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂), thetemperature of the shorted point immediately after the penetration ofthe nail did not reach 200° C. Comparison of the battery voltage betweenbefore and after the nail penetration test showed that the batteryvoltage after the test was almost the same as before. The maximumtemperature of the battery surface (i.e., the area apart from theshorted point) did not reach 80° C. throughout the test. The foregoingindicates that the shorted point was successfully insulated after theoccurrence of internal short-circuit, which minimized the Joule heatgeneration.

From the above results, it is clear that when the positive electrodematerial has a certain thermal stability (i.e., an estimated heatgeneration rate at 200° C. determined by the ARC measurement of 10W/kg), the battery can ensure safety even under the conditions wheremultiple simultaneous internal short-circuits occur. This is caused bythe synergistic effect between the action of the heat-resistantinsulating layer and the thermal stability of the positive electrodematerial. Presumably, this synergistic effect contributed to theachievement of a battery with an unprecedented level of safety. Althoughit has been considered impossible to reduce the battery temperature toless than 80° C. under the conditions where multiple shorted pointsoccur simultaneously, the foregoing has clearly illustrated that it ispossible according to the present invention.

Hereinafter, a description will be given of examples.

Batteries X1 to X18 and Batteries Y1 to Y12

Thirty different 18650 type cylindrical non-aqueous electrolytesecondary batteries (hereinafter referred to as batteries X1 to X18 andY1 to Y12) were produced in the same manner as in EXPERIMENT 1 exceptthat the positive electrode materials listed in Table 5 were used, thatthe heat-resistant insulating layer was formed on both surfaces (exceptBatteries X7 to X12, X16 to X18, Y3, Y4, Y8, Y11 and Y12) of those aslisted in Table 5, and that the thickness of the dried heat-resistantinsulating layer formed on each surface was set as listed in Table 5.Note that the positive electrode materials 1 to 3 had compositions shownin Table 2, and that the positive electrode materials A to E hadcompositions shown below.

Positive electrode material A: a mixture containing 90 wt % of positiveelectrode material 1 and 10 wt % of positive electrode material 2.

Positive electrode material B: a mixture containing 80 wt % of positiveelectrode material 1 and 20 wt % of positive electrode material 2.

Positive electrode material C: a mixture containing 70 wt % of positiveelectrode material 1 and 30 wt % of positive electrode material 2.

Positive electrode material D: a mixture containing 60 wt % of positiveelectrode material 1 and 40 wt % of positive electrode material 2.

Positive electrode material E: a mixture containing 50 wt % of positiveelectrode material 1 and 50 wt % of positive electrode material 2.

In the production of batteries X7 to X12, X16 to X18, Y3, Y4, Y8, Y11and Y12, as the heat-resistant insulating layer, a 0.5 to 5 μm thickfilm made of aramid resin and an inorganic oxide filler as disclosed byJapanese Laid-Open Patent Publication No. Hei 9-208736 was formed ononly one surface of a separator. The formation of the heat-resistantinsulating layer is described in detail below.

A separable flask equipped with a stirring blade, a thermometer, anitrogen feed pipe and a powder inlet was thoroughly dried. Into thedried separable flask was introduced 4200 g of NMP and 270 g of calciumchloride having dried at 200° C. for two hours, which was then heated to100° C. After the calcium chloride was thoroughly dissolved, thetemperature in the flask was decreased back to 20±2° C., and 130 g ofpara-phenylenediamine (PPD) was added and dissolved thoroughly. Then, 24g of dichloroterephthate (TPC) was added to the solution, which was keptat 20±2° C., every five minutes (10 additions and 240 g in total). Theresulting solution was allowed to sit for one hour for aging, which wasthen stirred for 30 minutes under a reduced pressure for degassing toobtain a polymer solution of polyparaphenylene terephthalamide (PPTA:with a thermal decomposition temperature of not less than 400° C.,amorphous).

To the polymer solution was slowly added an NMP solution having 5.8 wt %calcium chloride dissolved therein until the concentration of PPTAreached 2.8 wt %. Alumina particles having an average particle size of0.5 μm were further added thereto to prepare a paste comprising PPTAsolution and alumina at a weight ratio of 97:3. The obtained paste wasapplied onto one surface of a separator using a bar coater, which wasthen dried with hot air at 80° C. The resulting separator was washedwith ion exchange water to remove calcium chloride. Thereby, a separatorhaving a heat-resistant insulating layer comprising PPTA was produced.In the production of an electrode assembly, the heat-resistantinsulating layer was disposed such that the layer came in contact withthe positive electrode.

In the production of batteries Y5 and Y6, no heat-resistant insulatinglayer was formed.

In the same manner as in EXPERIMENT 2, the estimated heat generationrate at 200° C. of the positive electrode material mixture for thebatteries X1 to X18 and Y1 to Y12 was determined. The results are shownin Table 5.

Further, the batteries X1 to X18 and Y1 to Y12, ten of each, weresubjected to the same nail penetration test as in EXPERIMENT 1 to checkthe maximum battery surface temperature apart from the shorted point.With respect to the ten batteries of each EXAMPLE, the average of themaximum battery surface temperature of batteries whose temperature didnot reach 80° C. was calculated, and the number of batteries whosetemperature reached 80° C. was counted. When the maximum battery surfacetemperatures of the ten batteries were all 80° C. or higher, they areexpressed as “not less than 80” in Table 5. The results are shown inTable 5. TABLE 5 Heat- Estimated resistant Number of Maximum heatinsulating batteries battery Positive generation Location where heat-layer that surface electrode rate resistant insulating layer thicknessreached temperature material (W/kg) is formed (μm) 80° C. (° C.) BatteryY1 1 10 Negative electrode surface 0.5 3 52 Battery X1 1 10 Negativeelectrode surface 1 1 33 Battery X2 1 10 Negative electrode surface 2.51 33 Battery X3 1 10 Negative electrode surface 3 0 30 Battery X4 1 10Negative electrode surface 5 0 30 Battery X5 1 10 Negative electrodesurface 10 0 29 Battery X6 1 10 Negative electrode surface 15 0 29Battery Y2 1 10 Negative electrode surface 17 3 45 Battery Y3 1 10Separator surface 0.5 3 58 Battery X7 1 10 Separator surface 1 1 39Battery X8 1 10 Separator surface 1.7 0 36 Battery X9 1 10 Separatorsurface 5 0 35 Battery X10 1 10 Separator surface 6.7 0 36 Battery X11 110 Separator surface 10 1 38 Battery X12 1 10 Separator surface 15 1 40Battery Y4 1 10 Separator surface 17 3 54 Battery X13 A 15 Negativeelectrode surface 5 0 35 Battery X14 B 33 Negative electrode surface 5 038 Battery X15 C 46 Negative electrode surface 5 0 42 Battery X16 A 15Separator surface 5 0 39 Battery X17 B 33 Separator surface 5 0 42Battery X18 C 46 Separator surface 5 0 43 Battery Y5 2 100 Not formed —10 not less than 80 Battery Y6 1 10 Not formed — 10 not less than 80Battery Y7 3 2500 Negative electrode surface 5 10 not less than 80Battery Y8 3 2500 Separator surface 5 10 not less than 80 Battery Y9 D61 Negative electrode surface 5 10 not less than 80 Battery Y10 E 72Negative electrode surface 5 10 not less than 80 Battery Y11 D 61Separator surface 5 10 not less than 80 Battery Y12 E 72 Separatorsurface 5 10 not less than 80Positive electrode material 1: LiMn_(1/3)Ni_(1/3)Co_(1/3)O₂Positive electrode material 2: LiAl_(0.05)Ni_(0.8)Co_(0.15)O₂Positive electrode material 3: LiCoO₂Positive electrode material A: Positive electrode material 1/Positiveelectrode material 2 = 90/10 (wt %)Positive electrode material B: Positive electrode material 1/Positiveelectrode material 2 = 80/20 (wt %)Positive electrode material C: Positive electrode material 1/Positiveelectrode material 2 = 70/30 (wt %)Positive electrode material D: Positive electrode material 1/Positiveelectrode material 2 = 60/40 (wt %)Positive electrode material E: Positive electrode material 1/Positiveelectrode material 2 = 50/50 (wt %)

The test results are discussed below.

The results of Batteries Y5 and Y6 indicate that, even when the positiveelectrode active material having relatively higher thermal stabilitythan lithium cobalt oxide is used, unless the heat-resistant insulatinglayer is formed, the maximum battery surface temperature cannot bereduced to less than 80° C. under the conditions where multiplesimultaneous internal short-circuits occur.

As can be seen from the results of Batteries Y7 to Y12, even the batteryhaving the heat-resistant insulating layer cannot interrupt the chain ofheat generation reaction when the battery contains a positive electrodeactive material having an estimated heat generation rate of over 50W/kg. Thus, the maximum battery surface temperature cannot be reduced toless than 80° C.

Comparisons between the results of Batteries X13 to X18 and those ofBatteries Y9 to Y12 show that the heat-resistant insulating layer canexhibit its action in the most effective manner when a positiveelectrode active material having an estimated heat generation rate ofnot greater than 50 W/kg is used. When the estimated heat generationrate is greater than 50 W/kg, the maximum battery surface temperatureincreases significantly. When the estimated heat generation rate is notgreater than 50 W/kg, the chain of heat generation reaction can beeffectively interrupted, and the heat generated at the shorted point canbe diffused efficiently.

It can be seen from the results of Batteries X1 to X6 that if theheat-resistant insulating layer has a certain thickness, the thicknessof the heat-resistant insulating layer does not significantly affect theeffect of reducing the maximum battery surface temperature under theconditions where multiple simultaneous short-circuits occur. The maximumbattery surface temperature decreased as the thickness of theheat-resistant insulating layer was increased. However, when theheat-resistant insulating layer is too thick, it will be difficult tomaintain a high energy density of the battery, and the heat-resistantinsulating layer may break upon winding. Since the heat-resistantinsulating layer is brittle, if it is too thick, it partially separatesfrom the electrode surface or the separator surface upon winding. Thiscan be confirmed from the fact that the number of batteries whosetemperature reached 80° C or higher was particularly large in BatteryY2. Thus, even when a positive electrode active material with highthermal stability is used, a high degree of safety cannot be maintainedin nail penetration test.

Accordingly, the thickness of the heat-resistant insulating layer maybe, for example, about 1 to 15 μm, or 3 to 10 μm. Similar results werealso obtained in Batteries X7 to X12 in which the heat-resistantinsulating layer was formed on the separator surface. The thickness ofthe heat-resistant insulating layer containing aramid resin may be, forexample, 1.7 to 6.7 μm.

When the thickness of the heat-resistant insulating layer is less than 1μm, the mechanical strength thereof will be low, which means that theheat-resistant insulating layer can be easily damaged by the impactassociated with a short-circuit. This can be confirmed from the factthat the number of batteries whose temperature reached 80° C. or higherwas particularly large in Batter Y1. Accordingly, when the thickness ofthe heat-resistant insulating layer is reduced to less than 1 μm, theinsulation function will degrade to a certain extent.

Batteries X19A to X30A and Battery Y13A

Thirteen different 18650 type cylindrical non-aqueous electrolytesecondary batteries (hereinafter referred to as batteries X19A to X30Aand Battery Y13A were produced in the same manner as in EXPERIMENT 1except that the positive electrode materials listed in Table 6-1 wereused, that the heat-resistant insulating layer was formed on bothsurfaces of those listed in Table 6-1, and that the thickness of thedried heat-resistant insulating layer formed on each surface was set tothose listed in Table 6-1.

Batteries X19B to X30B and Battery Y13B

Thirteen different 18650 type cylindrical non-aqueous electrolytesecondary batteries (hereinafter referred to as batteries X19B to X30Band Batteries Y13B) were produced in the same manner as in EXPERIMENT 1except that the positive electrode materials as listed in Table 6-2 wereused, that the heat-resistant insulating layer was formed as listed inTable 6-2, that the thickness of the dried heat-resistant insulatinglayer was set to those listed in Table 6-2, and that a 5-μm-thick filmcomprising aramid resin and an inorganic oxide filler, which wasdisclosed in the EXAMPLE of Japanese Laid-Open Patent Publication No.Hei 9-208736, was formed on only one surface of the separator as theheat-resistant insulating layer as in Battery X9.

Note that, as the positive electrode materials 4 to 13, compositelithium oxides having compositions (LiM_(b)Ni_(d)Co_(e)O₂) shown inTable 7 were used, and that the positive electrode materials F to H hadcompositions shown below.

Positive electrode material F: a mixture containing 90 wt % of positiveelectrode material 1 and 10 wt % of positive electrode material 3.

Positive electrode material G: a mixture containing 80 wt % of positiveelectrode material 1 and 20 wt % of positive electrode material 3.

Positive electrode material H: a mixture containing 70 wt % of positiveelectrode material 1 and 30 wt % of positive ctrode material 3. TABLE6-1 Heat- resistant Maximum Estimated insulating battery Positive heatLocation where heat- layer surface electrode generation resistantinsulating thickness temperature Battery material rate (W/kg) layer isformed (μm) (° C.) X19A F 24 Negative electrode surface 5 55 X20A G 49Negative electrode surface 5 62 X21A 4 47 Negative electrode surface 561 X22A 5 35 Negative electrode surface 5 57 X23A 6 40 Negativeelectrode surface 5 59 X24A 7 41 Negative electrode surface 5 59 X25A 834 Negative electrode surface 5 57 X26A 9 37 Negative electrode surface5 57 X27A 10 38 Negative electrode surface 5 58 X28A 11 45 Negativeelectrode surface 5 61 X29A 12 42 Negative electrode surface 5 60 X30A13 42 Negative electrode surface 5 60 Y13A H 72 Negative electrodesurface 5 not less than 80

TABLE 6-2 Heat- Maximum Estimated Location resistant battery Positiveheat where heat- insulating surface elec- generation resistant layertem- trode rate insulating thickness perature Battery material (W/kg)layer is formed (μm) (° C.) X19B F 24 Separator surface 5 59 X20B G 49Separator surface 5 67 X21B 4 47 Separator surface 5 65 X22B 5 35Separator surface 5 63 X23B 6 40 Separator surface 5 63 X24B 7 41Separator surface 5 62 X25B 8 34 Separator surface 5 61 X26B 9 37Separator surface 5 62 X27B 10 38 Separator surface 5 62 X28B 11 45Separator surface 5 66 X29B 12 42 Separator surface 5 64 X30B 13 42Separator surface 5 65 Y13B H 72 Separator surface 5 not less than 80

TABLE 7 Composition LiMn_(0.27)M¹ _(0.03)Ni_(0.5)Co_(0.2)O₂ Atomic ratio0.27 0.03 0.5 0.2 Positive electrode Mn Mn Ni Co material 4 Positiveelectrode Mn Al Ni Co material 5 Positive electrode Mn Cu Ni Co material6 Positive electrode Mn Mg Ni Co material 7 Positive electrode Mn Ti NiCo material 8 Positive electrode Mn Zn Ni Co material 9 Positiveelectrode Mn Mo Ni Co material 10 Positive electrode Mn Sn Ni Comaterial 11 Positive electrode Mn In Ni Co material 12 Positiveelectrode Mn Fe Ni Co material 13

In the same manner as in EXPERIMENT 2, the estimated heat generationrate at 200° C. of the positive electrode material mixture wasdetermined. The results are shown in Tables 6-1 and 6-2.

Further, the batteries X19A to X30A, X19B to X30B, Y13A and Y13B, ten ofeach, were subjected to the same nail penetration test as in EXPERIMENT1 to check the maximum battery surface temperature. The average of themaximum battery surface temperature of ten batteries was calculated. Theresults are shown in Tables 6-1 and 6-2. The maximum battery surfacetemperatures of the ten batteries were all below 80° C.

The test results are discussed below.

The comparison of the results of Battery X21A and Batteries X22A to X30Aand the comparison of the results of Battery X21B and Batteries X22B toX30B indicates that, Al, Sn, In, Fe, Cu, Mg, Ti, Zn and Mo have theeffect of reducing the estimated heat generation rate. When using acomposite lithium oxide comprising Mn (the element M) and other elementM¹ as listed in Table 7, the molar ratio (atomic ratio) of Mn and theelement M¹ is preferably 99:1 to 50:50, more preferably 97:3 to 90:10.

The addition of lithium cobalt oxide (i.e., positive electrode material3) to the active material produces a high energy density positiveelectrode, and therefore the addition of lithium cobalt oxide ispreferred from the view point of achieving a high capacity battery.However, batteries using the positive electrode material H, 30 wt % ofwhich was positive electrode material 3, were unsatisfactory in terms ofsafety in the nail penetration test. Accordingly, when lithium cobaltoxide is used together with other positive electrode material, theamount of lithium cobalt oxide is preferably not greater than 20 wt % ofthe total amount of the active material.

Batteries X31A to X41A

Eleven different 18650 type cylindrical non-aqueous electrolytesecondary batteries (hereinafter referred to as batteries X31A to X41A)were produced in the same manner as in EXPERIMENT 1 except that thepositive electrode materials 14 to 24 (composite lithium oxides having acomposition of LiCo_(0.98)M_(0.02)O₂) listed in Table 8-1 were used, andthat a heat-resistant insulating layer comprising aramid resin and aninorganic oxide filler was formed on a separator such that the driedlayer had a thickness of 5 μm as in Battery X9. In the same manner as inEXPERIMENT 2, the estimated heat generation rate at 200° C. of thepositive electrode material mixture was determined. Further, thebatteries X31A to X41A, ten of each, were subjected to the nailpenetration test, and the average of the maximum battery surfacetemperature of ten batteries was calculated. The results are shown inTable 8-1. The maximum battery surface temperatures of the ten batterieswere all below 80° C.

Batteries X31B to X41B

Eleven different 18650 type cylindrical non-aqueous electrolytesecondary batteries (hereinafter referred to as batteries X31B to X41B)were produced in the same manner as in EXPERIMENT 1 except that thepositive electrode materials 14 to 24 (composite lithium oxides having acomposition of LiCo_(0.98)M_(0.02)O₂) listed in Table 8-2 were used, andthat a heat-resistant insulating layer comprising an inorganic oxidefiller and BM-720H was formed on both surfaces of the negative electrodesuch that the dried layer had a thickness of 5 μm as in Battery X4. Inthe same manner as in EXPERIMENT 2, the estimated heat generation rateat 200° C. of the positive electrode material mixture was determined.Further, the batteries X31B to X41B, ten of each, were subjected to thenail penetration test, and the average of the maximum battery surfacetemperature of ten batteries was calculated. The results are shown inTable 8-2. The maximum battery surface temperatures of the ten batterieswere all below 80° C. TABLE 8-1 Estimated Maximum heat batterygeneration surface Composition LiCo_(0.98)M_(0.02)O₂ rate temperatureAtomic ratio 0.98 0.02 (W/kg) (° C.) X31A Positive electrode Co Mn 47 70material 14 X32A Positive electrode Co Al 42 63 material 15 X33APositive electrode Co Cu 46 65 material 16 X34A Positive electrode Co Mg46 65 material 17 X35A Positive electrode Co Ti 38 61 material 18 X36APositive electrode Co Zn 42 63 material 19 X37A Positive electrode Co Mo43 65 material 20 X38A Positive electrode Co Sn 47 69 material 21 X39APositive electrode Co In 47 67 material 22 X40A Positive electrode Co Fe46 67 material 23 X41A Positive electrode Co Zr 36 60 material 24

TABLE 8-2 Estimated Maximum heat battery generation surface CompositionLiCo_(0.98)M_(0.02)O₂ rate temperature Atomic ratio 0.98 0.02 (W/kg) (°C.) X31B Positive electrode Co Mn 47 67 material 14 X32B Positiveelectrode Co Al 42 61 material 15 X33B Positive electrode Co Cu 46 60material 16 X34B Positive electrode Co Mg 46 61 material 17 X35BPositive electrode Co Ti 38 58 material 18 X36B Positive electrode Co Zn42 60 material 19 X37B Positive electrode Co Mo 43 61 material 20 X38BPositive electrode Co Sn 47 65 material 21 X39B Positive electrode Co In47 64 material 22 X40B Positive electrode Co Fe 46 65 material 23 X41BPositive electrode Co Zr 36 58 material 24

The results of Table 8 show that Mn, Al, Sn, In, Fe, Cu, Mg, Ti, Zn, Zrand Mo have the effect of reducing the estimated heat generation rate.Even when the composition was based on the positive electrode material3, the addition of the element M reduced the estimated heat generationrate at 200° C. to not greater than 50 W/kg. Moreover, the synergisticeffect between the element M and the heat-resistant insulating layersignificantly improved the nail penetration safety.

Batteries X42A to X52A

Eleven different 18650 type cylindrical non-aqueous electrolytesecondary batteries (hereinafter referred to as batteries X42A to X52A)were produced in the same manner as in EXPERIMENT 1 except that thepositive electrode materials 25 to 35 listed in Table 9-1 were used, andthat a heat-resistant insulating layer comprising aramid resin and aninorganic oxide filler was formed on a separator such that the driedlayer had a thickness of 5 μm as in Battery X9. In the same manner as inEXPERIMENT 2, the estimated heat generation rate at 200° C. of thepositive electrode material mixture was determined. Further, thebatteries X42A to X52A, ten of each, were subjected to the nailpenetration test, and the average of the maximum battery surfacetemperature of ten batteries was calculated. The results are shown inTable 9-1. The maximum battery surface temperatures of the ten batterieswere all below 80° C.

Batteries X42B to X52B

Eleven different 18650 type cylindrical non-aqueous electrolytesecondary batteries (hereinafter referred to as batteries X42B to X52B)were produced in the same manner as in EXPERIMENT 1 except that thepositive electrode materials 25 to 35 listed in Table 9-2 were used, andthat a heat-resistant insulating layer comprising an inorganic oxidefiller and BM-720H was formed on both surfaces of the negative electrodesuch that the dried layer had a thickness of 5 μm as in Battery X4. Inthe same manner as in EXPERIMENT 2, the estimated heat generation rateat 200° C. of the positive electrode material mixture was determined.Further, the batteries X42B to X52B, ten of each, were subjected to thenail penetration test, and the average of the maximum battery surfacetemperature of ten batteries was calculated. The results are shown inTable 9-2. The maximum battery surface temperatures of the ten batterieswere all below 80° C.

The positive electrode materials 25 to 34 were each prepared by mixingthe positive electrode material 2 (LiAl_(0.05)Ni_(0.8)Co_(0.15)O₂) withan oxide of the element M shown in Table 9, followed by baking at 1000°C. in air atmosphere. The amount of the oxide of the element M was 0.01mol relative to 1 mol of the positive electrode material 2. As a result,the element M diffused from the added oxide to the positive electrodematerial 2, and positive electrode materials 25 to 34 comprisingcomposite lithium oxides in which the element M was distributed more inthe surface portion than the inside were obtained.

The positive electrode material 35 was prepared by treating the positiveelectrode material 2 with vinyl trimethoxy silane serving as a silanecoupling agent. In this example, the positive electrode material 2 wasfirst impregnated with a mixture of the silane coupling agent and waterat a silane coupling agent concentration of 0.1 wt %, and then dried.TABLE 9-1 Maximum Estimated battery heat surfaceLiAl_(0.05)Ni_(0.8)Co_(0.15)O₂ generation temperature Composition Addedelement rate (W/kg) (° C.) X42A Positive electrode Mn(MnO₄) 44 69material 25 X43A Positive electrode Cu(CuO) 40 66 material 26 X44APositive electrode Mg(MgO) 42 65 material 27 X45A Positive electrodeTi(TiO) 34 61 material 28 X46A Positive electrode Zn(ZnO) 37 63 material29 X47A Positive electrode Mo(MoO₂) 39 65 material 30 X48A Positiveelectrode Sn(SnO) 43 67 material 31 X49A Positive electrode In(In₂O₃) 4267 material 32 X50A Positive electrode Fe(Fe₂O₃) 45 68 material 33 X51APositive electrode Zr(ZrO) 32 59 material 34 X52A Positive electrode Sicompound 31 54 material 35

TABLE 9-2 Maximum Estimated battery heat surfaceLiAl_(0.05)Ni_(0.8)Co_(0.15)O₂ generation temperature Composition Addedelement rate (W/kg) (° C.) X42B Positive electrode Mn(MnO₄) 44 66material 25 X43B Positive electrode Cu(CuO) 40 64 material 26 X44BPositive electrode Mg(MgO) 42 61 material 27 X45B Positive electrodeTi(TiO) 34 58 material 28 X46B Positive electrode Zn(ZnO) 37 60 material29 X47B Positive electrode Mo(MoO₂) 39 61 material 30 X48B Positiveelectrode Sn(SnO) 43 64 material 31 X49B Positive electrode In(In₂O₃) 4263 material 32 X50B Positive electrode Fe(Fe₂O₃) 45 64 material 33 X51BPositive electrode Zr(ZrO) 32 56 material 34 X52B Positive electrode Sicompound 31 50 material 35

The results of Tables 9-1 and 9-2 also show that the element M has theeffect of reducing the estimated heat generation rate, in the samemanner as Tables 8-1 and 8-2. Because the element M was distributed witha higher concentration in the surface portion of the active material,the effect of reducing the estimated heat generation rate wasremarkable. Moreover, the synergistic effect between the element M andthe heat-resistant insulating layer significantly improved the nailpenetration safety. Similar results to those obtained by the addition ofthe element M were obtained by the treatment with the silane couplingagent.

Batteries X53A to X55A

Three different 18650 type cylindrical non-aqueous electrolyte secondarybatteries (hereinafter referred to as batteries X53A to X55A) wereproduced in the same manner as in EXPERIMENT 1 except that positiveelectrode materials 36 to 38 prepared by mixing the positive electrodematerial 1 and the positive electrode material 24 shown below were used,and that a heat-resistant insulating layer comprising aramid resin andan inorganic oxide filler was formed on a separator such that the driedlayer had a thickness of 5 μm as in Battery X9. In the same manner as inEXPERIMENT 2, the estimated heat generation rate at 200° C. of thepositive electrode material mixture was determined. Further, thebatteries X53A to X55A, ten of each, were subjected to the nailpenetration test, and the average of the maximum battery surfacetemperature of ten batteries was calculated. The results are shown inTable 10-1. The maximum battery surface temperatures of the tenbatteries were all below 80° C.

Batteries X53B to X55B

Three different 18650 type cylindrical non-aqueous electrolyte secondarybatteries (hereinafter referred to as batteries X53B to X55B) wereproduced in the same manner as in EXPERIMENT 1 except that positiveelectrode materials 36 to 38 prepared by mixing the positive electrodematerials 1 and 24 together were used, and that a heat-resistantinsulating layer comprising an inorganic oxide filler and BM-720H wasformed on both surfaces of the negative electrode such that the driedlayer had a thickness of 5 μm as in Battery X4. In the same manner as inEXPERIMENT 2, the estimated heat generation rate at 200° C. of thepositive electrode material mixture was determined. Further, thebatteries X53B to X55B, ten of each, were subjected to the nailpenetration test, and the average of the maximum battery surfacetemperature of ten batteries was calculated. The results are shown inTable 10-2. The maximum battery surface temperatures of the tenbatteries were all below 80° C.

Positive electrode material 36: a mixture containing 10 wt % of positiveelectrode material 1 and 90 wt % of positive electrode material 24.

Positive electrode material 37: a mixture containing 50 wt % of positiveelectrode material 1 and 50 wt % of positive electrode material 24.

Positive electrode material 38: a mixture containing 90 wt % of positiveelectrode material 1 and 10 wt % of positive electrode material 24.TABLE 10-1 Maximum battery Estimated heat surface generation temperaturerate (W/kg) (° C.) X53A Positive 33 58 electrode material 36 X54APositive 23 42 electrode material 37 X55A Positive 13 31 electrodematerial 38

TABLE 10-2 Maximum battery Estimated heat surface generation temperaturerate (W/kg) (° C.) X53B Positive 33 55 electrode material 36 X54BPositive 23 38 electrode material 37 X55B Positive 13 29 electrodematerial 38

It is clear from Tables 10-1 and 10-2 that even when two differentpositive electrode materials, both having a reduced estimatedheat-generation rate of not greater than 50 W/kg, are used together, thenail penetration safety can be improved significantly.

Batteries X56, X57 and X59

Three different 18650 type cylindrical non-aqueous electrolyte secondarybatteries (hereinafter referred to as batteries X56, X57 and X59) wereproduced in the same manner as in EXPERIMENT 1 except that theheat-resistant insulating layer was formed on those listed in Table 11.Note that when the heat-resistant insulating layer was formed on aseparator, the layer was formed on either the surface of the separatorto be in contact with the positive electrode or the surface of theseparator to be in contact with the negative electrode as shown in Table11.

Battery X58

A battery was produced in the same manner as in Battery X9 except that,instead of aramid resin, polyamide imide resin (with a thermaldecomposition temperature of not less than 400° C. and a glasstransition temperature of 250° C., amorphous) was used.

Battery X60

A battery was produced in the same manner as in Battery X9 except thatthe heat-resistant insulating layer was formed on the surface of theseparator to be in contact with the negative electrode.

Battery X61

A battery was produced in the same manner as in Battery X58 except thatthe heat-resistant insulating layer was formed on the surface of theseparator to be in contact with the negative electrode.

Battery X62

A battery was produced in the same manner as in Battery Y6 except that a5 μm thick heat-resistant insulating layer sheet, which was independentof the positive electrode, the negative electrode and the separator, wasproduced by applying the paste for the heat-resistant insulating layeronto a fluorocarbon resin sheet, which was then dried, and the resultingsheet was peeled, and that the obtained heat-resistant insulating layersheet was placed between the positive electrode and the separator.

Battery X63

A battery was produced in the same manner as in Battery X62 except thata heat-resistant insulating layer sheet comprising polyamide resinhaving the same composition as used in Battery X9 was prepared in thesame manner as in Battery X62 and was used.

Battery X64

A battery was produced in the same manner as in Battery X62 except thata heat-resistant insulating layer sheet comprising polyamide imide (PAI)resin having the same composition as used in Battery X58 was prepared inthe same manner as in Battery X62 and was used.

Battery X65

A battery was produced in the same manner as in Battery X4 except that,as the inorganic oxide filler for the heat-resistant insulating layer,magnesia (magnesium oxide) having a median size of 0.3 μm was used,instead of alumina having a median size of 0.3 μm.

Battery X66

A battery was produced in the same manner as in Battery X56 except that,as the inorganic oxide filler for the heat-resistant insulating layer,magnesia (magnesium oxide) having a median size of 0.3 μm was used,instead of alumina having a median size of 0.3 μm.

The batteries X56 to X66, ten of each, were subjected to the same nailpenetration test as in EXPERIMENT 1, and the average of the maximumbattery surface temperature of ten batteries was calculated. The resultsare shown in Table 11. TABLE 11 Location where heat-resistant insulatinglayer is formed Location Surface with Maximum Material where heat- whichheat- battery Positive of Heat- resistant resistant surface electroderesistant insulating insulating tem- material insulating layer layer isin perature Battery used layer is formed contact (° C.) X4 1 AluminaNegative — 30 electrode X56 1 Alumina Positive — 43 electrode X57 1Alumina Separator Positive 29 electrode surface X9 1 Aramid SeparatorPositive 28 electrode surface X58 1 PAI Separator Positive 29 electrodesurface X59 1 Alumina Separator Negative 29 electrode surface X60 1Aramid Separator Negative 29 electrode surface X61 1 PAI SeparatorNegative 29 electrode surface X62 1 Alumina Independent Positive 49electrode surface X63 1 Aramid Independent Positive 45 electrode surfaceX64 1 PAI Independent Positive 46 electrode surface X65 1 MagnesiaNegative — — electrode X66 1 Magnesia Positive — — electrodePAI: Polyamide imide

The test results are discussed below.

As can be seen from Table 11, regardless of which material is used toform the heat-resistant insulating layer, the safety in the nailpenetration test was improved. This clearly indicates that as long as amaterial having heat resistance and insulation capability is used, asimilar result can be obtained. It is further clear that the effectincreases when the heat-resistant insulating layer is formed on theseparator or the negative electrode. It is also clear that similarresults can be obtained using magnesia, instead of alumina.

In the above examples, cylindrical non-aqueous electrolyte secondarybatteries were produced, but it is to be understood that the battery ofthe present invention is not limited to a cylindrical shape. Similarly,a carbon material was used as the negative electrode active material,and the results when the battery was charged up to the voltage of 4.25 vwere shown, even when an Si alloy or Sn alloy is used, the effect ofimproving safety can be obtained. Also, by using a positive electrodematerial mixture having an estimated heat generation rate of not greaterthan 50 W/kg together with the heat-resistant insulating layer, evenwhen the battery is charged to a higher voltage range (4.2 V to 4.6 V),the effect of improving safety can be obtained.

The non-aqueous electrolyte secondary battery of the present inventionhas a high energy density and a high level of safety, so that it ishighly applicable to the power sources for portable devices such aspersonal digital assistants (PDAs) and mobile electronic devices.However, the lithium secondary battery of the present invention can alsobe used for, for example, compact home electrical energy storagedevices, and the power sources for motorcycles, electric cars and hybridelectric cars, and there is no particular limitation with respect to itsuse. While there is no particular limitation with respect to the shapeof the lithium ion secondary battery of the present invention, acylindrical shape and a square shape are preferable, for example. Thelithium secondary battery of the present invention has high outputcharacteristics, so that it is highly applicable to the power sourcesfor PDAs, electric operated tools, personal computers (PCs), electricoperated toys or electric operated robots, and large scale back-up powersources, uninterruptible power supplies (UPS), load leveling powersource system utilizing natural energy or regenerative energyutilization system.

Although the present invention has been described in terms of thepresently preferred embodiments, it is to be understood that suchdisclosure is not to be interpreted as limiting. Various alterations andmodifications will no doubt become apparent to those skilled in the artto which the present invention pertains, after having read the abovedisclosure. Accordingly, it is intended that the appended claims beinterpreted as covering all alterations and modifications as fall withinthe true spirit and scope of the invention.

1. A non-aqueous electrolyte secondary battery comprising: (a) apositive electrode comprising a positive electrode material mixturecontaining a composite lithium oxide and a positive electrode currentcollector carrying said positive electrode material mixture; (b) anegative electrode comprising a material capable of absorbing anddesorbing lithium; (c) a separator interposed between said positiveelectrode and said negative electrode, said separator comprising apolyolefin resin; (d) a non-aqueous electrolyte; and (e) aheat-resistant insulating layer interposed between said positiveelectrode and said negative electrode, wherein said positive electrodeand said negative electrode are wound together with said separator andsaid heat-resistant insulating layer interposed therebetween, whereinsaid positive electrode material mixture has an estimated heatgeneration rate at 200° C. of not greater than 50 W/kg.
 2. Thenon-aqueous electrolyte secondary battery in accordance with claim 1,wherein said heat-resistant insulating layer has a thickness of 1 μm ormore and 15 μm or less.
 3. The non-aqueous electrolyte secondary batteryin accordance with claim 1, wherein said heat-resistant insulating layerhas a thickness of 1 μm or more and 5 μm or less.
 4. The non-aqueouselectrolyte secondary battery in accordance with claim 1, wherein saidestimated heat generation rate is determined by the steps of: (i)determining a relation between absolute temperature T and heatgeneration rate V of said positive electrode material mixture using anaccelerating rate calorimeter (ARC); (ii) plotting a relation betweenthe inverse of absolute temperature T as an X coordinate and thelogarithm of heat generation rate V as a Y coordinate according to theArrhenius law; (iii) obtaining an approximate straight line fitted tothe plotted points in a heat generation temperature range of T<200° C.(473 K); and (iv) extrapolating the obtained approximate straight lineto the temperature axis at T=200° C (473 K).
 5. The non-aqueouselectrolyte secondary battery in accordance with claim 1, wherein saidcomposite lithium oxide has a composition represented by a generalformula (1) : Li_(a)M_(b)Me_(c)O₂, where element M is at least oneselected from the group consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti,Zn, Zr and Mo, and element Me is at least one selected from the groupconsisting of Ni and Co, and where said general formula (1) satisfies:0.9<a<1.2; 0.02≦b≦0.5; 0.5≦c≦0.98; and 0.95≦b+c≦1.05.
 6. The non-aqueouselectrolyte secondary battery in accordance with claim 1, wherein saidcomposite lithium oxide has a composition represented by a generalformula (2) : Li_(a)M_(b)Ni_(c)Co_(e)O₂, where element M is at least oneselected from the group consisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti,Zn, Zr and Mo, and where said general formula (2) satisfies: 0.9<a<1.2;0.02≦b≦0.5; 0.1≦d≦0.5; 0.1≦e≦0.5; and 0.95≦b+d+e≦1.05.
 7. Thenon-aqueous electrolyte secondary battery in accordance with claim 6,wherein said general formula (2) satisfies: 0.15≦b≦0.4; 0.3≦d≦0.5; and0.15≦e≦0.4.
 8. The non-aqueous electrolyte secondary battery inaccordance with claim 1, wherein said composite lithium oxide compriseselement M, said element M being at least one selected from the groupconsisting of Al, Mn, Sn, In, Fe, Cu, Mg, Ti, Zn, Zr and Mo, and saidelement M being distributed more in a surface portion of said compositelithium oxide than the inside of said composite lithium oxide.
 9. Thenon-aqueous electrolyte secondary battery in accordance with claim 1,wherein said composite lithium oxide has been treated with an Sicompound represented by a general formula (3): X—Si—Y₃, where X includesa functional group reactive with said composite lithium oxide, and Yincludes a functional group comprising C, H, O, F or Si.